Methods for controlling a wind turbine connected to the utility grid, wind turbine and wind park

ABSTRACT

The invention relates to methods for controlling a wind turbine connected to the utility grid by detecting status of the utility grid, and controlling one or more rotor blades and/or emitted power to the grid in returning to the operational wind turbine settings of normal grid mode. The invention also relates to a wind turbine and a wind park comprising at least two wind turbines.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of pending Internationalpatent application PCT/DK2007/000404 filed on Sep. 12, 2007 whichdesignates the United States and claims priority from Danish patentapplication PA 2006 01186 filed on Sep. 14, 2006, the content of whichis incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to methods for controlling a wind turbineconnected to the utility grid, a wind turbine and a wind park.

BACKGROUND OF THE INVENTION

National utility grid companies sets out the strategies and requirementsfor the connection of power generation equipment to the utility grid.These connection requirements are detailed described in Grid Codes andvaries from nation to nation.

One of the topics discussed in the grid codes is the capabilities of awind turbine when the utility grid experiences a fault. It is essentialfor the operation of a wind turbine and for a reliable supply of powerthat a wind turbine can stay connected and synchronized to the utilitygrid during said grid fault.

System faults are typically short circuits and can be any combination ofa three phases and earth short circuits. When a short circuit occur theflow of current results in voltage drops (low voltage situation)throughout the utility grid and the magnitude is dependent of themagnitude of the fault current, the impedance of the short circuit pathand the type of short circuit. Further, a low voltage situation maycomprise more than one voltage drop e.g. two successive voltage drops.

When a wind turbine experience a utility grid fault the generator speedincreases almost immediately as a result of the excessive aerodynamicalpower that can not be converted to electrical power. Therefore theaerodynamical power must be reduced drastically throughout the period ofutility grid fault.

One method of prior art is to turn the blades of the wind turbine froman operating position to a park position and allow the wind turbinegenerator to trip offline when the utility grid fault occurs. But as thegrid codes typically sets up requirements for a low voltage ride through(LVRT) that requires the wind turbine generator to stay connected, saidmethod of prior art is not compatible with the grid codes.

U.S. Pat. No. 6,921,985 discloses a LVRT system for a wind turbineconnected to a utility grid. The blade pitch angle is varied when a lowvoltage is detected in order to maintain the rotor speed below an overspeed trip limit.

It is an object of the present invention to provide advantageous methodsof operating a wind turbine in returning to the operational wind turbinesettings of normal grid mode.

SUMMARY OF THE INVENTION

The invention provides a method for controlling a wind turbine connectedto the utility grid comprising steps of:

-   -   detecting status of the utility grid,    -   detecting mechanical oscillations and/or loads of the wind        turbine, and    -   controlling one or more rotor blades and/or emitted power to the        grid in dependency of said mechanical oscillations and/or loads        in returning to the operational wind turbine settings of normal        grid mode.

Hereby it is ensured that when the rotor blade or blades are pitchedback to operational pitch angle settings of normal grid mode they can becontrolled in such a way that the hereby caused oscillations and/orloads will not significantly add to already existing oscillations and/orloads of the wind turbine.

In another aspect of the invention said detecting status of the utilitygrid comprises detecting at least one value indicating the status of theutility grid e.g. a normal grid mode or fault grid mode. Hereby it isensured that the wind turbine can return to operational wind turbinesettings as soon as possible after the recovery of a grid fault such asa low voltage grid fault.

In another aspect of the invention said returning to the operationalwind turbine settings comprises returning to normal grid mode from afault of the utility grid.

In another aspect of the invention returning to the operating settingsof normal grid mode comprises reapplying of thrust in counter phase tosaid mechanical oscillations and/or loads of the wind turbine e.g. thetower oscillation, rotor blade oscillation, foundation oscillation orcombinations hereof.

Hereby it is ensured that oscillations and/or loads produced in the windturbine by returning to operating settings of normal grid mode does notgive cause to constructive interference of said oscillations and/orloads. Constructive interference can cause excessive loads on said windturbine that will exceed load limits and result in damage to the windturbine.

Furthermore it is ensured that oscillations and/or loads produced in thewind turbine by returning to operating settings of normal grid mode cancause destructive interference of said oscillations and/or loads,whereby said oscillations is damped.

In another aspect of the invention said mechanical oscillations and/orloads is detected by means located in the wind turbine e.g. in thetower, nacelle, rotor blades and/or foundation. Hereby it is ensuredthat means to detect oscillations and/or loads are located in windturbine components that is under influence of said oscillations and/orloads whereby a more accurate detection can be obtained.

In another aspect of the invention said detecting status of the utilitygrid comprises detection of the grid voltage and/or the voltage/secslope of an alternating utility grid voltage. Hereby it is ensured thatvital parameters are detected in order to give a valid detection of saidstatus.

The invention also provides a method for controlling a wind turbineconnected to the utility grid comprising steps of:

-   -   detecting status of the utility grid, and    -   controlling one or more rotor blades and/or emitted power to the        grid in dependency of a fixed control value such as constant        pitch rate and/or predefined time period in returning to the        operating settings of normal grid mode.

Hereby it is ensured that when the rotor blade or blades are pitchedback to operational pitch angle settings of normal grid mode, the herebycaused oscillations and/or loads will not significantly add to alreadyexisting oscillations and/or of the wind turbine.

In another aspect of the invention said fixed control value issubstantial different from its extremes. By the term “extremes” is meantthe highest and/or lowest possible value of said control value. Herebyit is ensured that returning to the operating settings of normal gridmode is done by using control values that does not result in significantadditional loads on the wind turbine.

In another aspect of the invention returning to the operating settingsof normal grid mode comprises reapplying of thrust with a constant pitchrate such as less than 10 degree/sec. preferably 8 degree/sec. and/orover a predefined time period such as more than 0.5 seconds preferably 4seconds.

In another aspect of the invention returning to the operating settingsof normal grid mode comprises reapplying of thrust in dependency of thewind turbine tower eigenfrequency. Hereby it is ensured that reapplyingof thrust can be optimized to different types of tower whereby loads onsaid tower can be minimized.

In another aspect of the invention said detecting status of the utilitygrid comprises detection of the grid voltage and/or the voltage/secslope of an alternating utility grid voltage. Hereby it is ensured thatvital parameters are detected in order to give a valid detection of saidstatus.

The invention also relates to a wind turbine connected to a utility gridcomprising:

-   -   at least one blade pitch system to control the pitch of one or        more rotor blades, and    -   at least one wind turbine control system including a system        performing a method according to one or more of the preceding        claims in returning to the operating settings of normal grid        mode.

Hereby it is ensured that the wind turbine is controlled with controlvalues that are adapted to the specific operating situation whenreturning from a grid fault mode to a normal grid mode.

Even further the invention relates to a wind turbine further comprisingat least one detector for detecting failures in the utility grid e.g. atleast one voltage detector. Hereby an advantageous control of the windturbine is ensured.

The invention also relates to a wind park comprising at least two windturbines according to the wind turbine claim and at least one detectorfor detecting failures in the utility grid.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the following with reference to thefigures in which

FIG. 1 illustrates a large modern wind turbine including three windturbine blades in the wind turbine rotor,

FIG. 2 illustrates schematically a preferred embodiment of a windturbine with a control system for controlling the pitch angles of thewind turbine blades,

FIG. 3 illustrates the conceptual state-sequence for the presentinvention,

FIG. 4 illustrates two of at least three conditions that must be met inorder to detect a utility grid fault event,

FIG. 5 illustrates the conditions that must be met in order to detectthat the utility grid has recovered,

FIG. 6 illustrates a timing diagram for one preferred embodiment of theinvention

FIG. 7 illustrates a timing diagram for another preferred embodiment ofthe invention

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a modern wind turbine 1 with a tower 2 and a windturbine nacelle 3 positioned on top of the tower.

The wind turbine rotor, comprising at least one blade such as three windturbine blades 5 as illustrated, is connected to the hub 4 through pitchmechanisms 4 a. Each pitch mechanism includes a blade bearing and pitchactuating means which allows the blade to pitch. The pitch process iscontrolled by a pitch controller.

As illustrated in the figure, wind over a certain level will activatethe rotor and allow it to rotate in a perpendicular direction to thewind. The rotation movement is converted to electric power which usuallyis supplied to the utility grid as will be known by skilled personswithin the area.

FIG. 2 illustrates schematically one preferred embodiment of a controlsystem for controlling the pitch angles of the wind turbine blades

Data of the wind turbine 1 are measured with sensor means 7 such aspitch position sensors, blade load sensors, rotor azimuth sensors, toweracceleration sensors etc. The measured sensor data are supplied tocomputing means 8 in order to convert the data to feedback signals. Thefeedback signals are used in various control systems e.g. the pitchcontrol system 9 for controlling the pitch angle by establishing controlvalues for controlling said at least one wind turbine blade 5.

The computing means 8 preferably includes a microprocessor and computerstorage means for continuous control of the said feedback signal.

As indicated by the dashed arrows at the nacelle 3 the wind turbinetower 2 can oscillate resulting in a displacement of said nacelle 3. Aswill be known by skilled persons within the area said tower canoscillate at its eigenfrequency e.g. as a result of a sudden change inthrust applied to the rotor. Said oscillation can result in excessiveloads on said tower and in worst case cause damage.

FIG. 3 illustrates for one preferred embodiment of the inventionschematically a conceptual state-sequence diagram for the inventedcontrol algorithm comprising steps of:

-   -   Normal operation (state 0)    -   detecting a utility grid fault event,    -   an initial control of a wind turbine 1 in order to stabilize the        wind turbine rotor speed with new control parameters as a        response to said utility grid fault event (state 1),    -   an intermediate control of the wind turbine at a stabilized        level during the fault event (state 2),    -   detecting a recovery of the grid, and    -   a final control of the wind turbine while returning to normal        operating conditions (state 3 or state 13). As indicated on the        figure state 3 is entered if means for measuring values of the        mechanical oscillations and/or loads are present (a preferred        embodiment). State 13 is entered if said means are not present.

For one embodiment of the invention comprising a wind turbine 1comprising sensor means for measuring values representing mechanicaloscillations and/or loads of the wind turbine, a description of eachstate and the state switch conditions between said states is:

State Action in state Switch condition (to go to next state) 0 Normaloperation If utility grid fault detected then switch to state 1. Initialcontrol 1 Estimation of no If “short dip” then switch to state 0.acceleration pitch If grid recovery has been detected before angle(NOPA). NOPA has been reached then switch to Pitch towards NOPA state 2.with predefined If estimated NOPA has been reached then controlparameters switch to state 2. e.g. pitch velocity. Intermediate 2 IfNOPA reached: If grid recovery is detected AND wind control Wind turbineturbine oscillations and/or loads indicate controller is loaded thatpitching in will occur in counter phase with new temporary with saidoscillations and/or loads then parameter settings switch to state 3. atan rotor over speed. If NOPA not reached: Wind turbine controller isloaded with pre- defined control parameters. Final control 3 Pitch backto pitch If actual pitch angle value = pitch angle angle value obtainedvalue obtained before detected grid fault before detected grid thenswitch to state 0. fault with predefined control parameters e.g. withmaximal pitch rate.

For another embodiment of the invention comprising wind turbine 1without sensor means for measuring values representing the wind turbinemechanical oscillations and/or loads, a description of each state andthe state switch conditions between said states is:

State Description Switch condition (to go to next state) 0 Normaloperation If utility grid fault detected then switch to state 1. Initialcontrol 1 Estimation of no If “short dip” then switch to state 0.acceleration pitch If grid recovery has been detected angle (NOPA).before NOPA has been reached then Pitch towards NOPA switch to state 2.with predefined If estimated NOPA has been reached control parametersthen switch to state 2. e.g. pitch velocity. Intermediate 2 If NOPAreached: If grid recovery is detected then switch to control Windturbine controller state 13. is loaded with new temporary parametersettings at a rotor over speed. If NOPA not reached: Wind turbinecontroller is loaded with pre- defined control parameters. Final control13 Pitch back to pitch If actual pitch angle value = pitch angle valueobtained before value obtained before detected grid fault detected gridfault with then switch to state 0. predefined control parameters e.g.fixed pitch rate.

-   -   Common for both embodiments are states 0, 1 and 2:

State 0:

-   -   The mode of normal operation.

If the generated power by the wind turbine is below a predefined limitsuch as 25% of nominal power, said fault mode will not be initiated upondetecting a grid fault, as the normal control algorithm will be able toavoid over speeding of the rotor during said grid fault event and willcontinue operating in normal mode since there is no imminent danger dueto the low power in the wind.

State 1 (Initial Control):

When a grid fault has been detected said state 1 is entered. The basisof this state of the invented control algorithm is to pitch one or morewind turbine rotor blades 5 out of the wind immediately after a fault onthe utility grid is detected in order to avoid over speeding of therotor due to excessive aerodynamically power acting on said rotor.

For one embodiment of the invention the wind turbine rotor blades 5 arepitched to a no acceleration pitch angle (NOPA) which is defined to bethe pitch angle that gives equilibrium between aerodynamically power andany wind turbine losses and electrical generated power, hence giving noor substantially no acceleration of the generator. For one embodimentNOPA is calculated immediately after the grid fault is detected by tablelookup in a Cp-table. For another embodiment NOPA is established bycalculation e.g. with a mathematical algorithm.

For one embodiment of the invention if the grid fault is sufficientlyshort (a short dip) to have only a low impact on the turbine load, ashort dip situation is detected and it is preferred to obtain normaloperation and active power production as before said short dip as soonas possible. Consequently for this embodiment the invented algorithm isable to determine the level of significant loads on the turbine and onthat basis determine if the grid fault control sequence can be quittedor it needs to be completed.

For another embodiment of the invention where the grid fault lasts toolong to be detected as a said short dip but a grid recovery occursbefore NOPA is reached, it is preferred to obtain normal operation andactive power production as before the grid fault as soon as possible.For this situation a direct jump to normal operation mode would have atoo high impact on the wind turbine. Consequently for this embodimentthe invented algorithm jumps directly to state 2 where predefinedcontrol parameters are re-obtained as explained below.

A good indicator of how much the turbine has been affected, is how muchsaid aero dynamical thrust has been reduced. Pitch angle, toweracceleration, tower load, time or combinations hereof can be fairassumptions herefore. For the example of pitch angle as said indicator,to determine when to use said short dip control strategy or to continuethe grid fault control strategy to the next state, said algorithmcontinuously supervises how far the actual pitch angle is from therecent pitch angle before the grid fault. Consequently if a gridrecovery is detected and the difference between the actual pitch angleand a recent pitch angle immediately before said grid fault exceed acertain predefined level, said control algorithm will continue the gridfault control algorithm. Otherwise the grid fault control algorithm willbe terminated as fast as possible by returning to state 0 i.e. settingthe references for e.g. pitch angle, power and generator rpm to thesettings immediately before detecting said grid fault.

State 2 (Intermediate Control):

The basis of this state of the invented control algorithm is to keep thewind turbine operating within a defined range controllable by the windturbine controller and connected to the utility grid until the grid hasrecovered.

For one embodiment when NOPA is reached, wind turbine control isinitialized with the present control settings as reference e.g.generator rpm and pitch angle, in order to keep the generator rpmconstant or nearly constant at a level above the nominal speed.

For one embodiment where pitching out has been stopped due to detectedgrid recovery before reaching said NOPA, said state 2 is initiated andpredefined control parameters are re-obtained.

The control sequence stays in this state with the present controlsettings at least until recovery of the utility grid has been detected.

For one preferred embodiment, when means for measuring valuesrepresenting the wind turbine mechanical oscillations and/or loads arepresent, a switch to the next state (state 3) can be initiated whenpitching in the rotor blades will occur in counter phase to themechanical oscillations and/or loads of the wind turbine e.g.oscillations resulting form a tower acceleration. I.e. pitching saidrotor blades will be done in such a way that the oscillations and/orloads that will be generated by pitching said rotor blades back tonormal operation is controlled and generated in counter phase to theexisting oscillations and/or loads causing a dampening of the summarizedoscillations and/or loads.

For another preferred embodiment, when means for measuring valuesrepresenting the wind turbine mechanical oscillations and/or loads arenot present, said switching to the next state (state 13) can beinitiated as soon as recovery of the utility grid has been detected.

State 3: (Final Control—if Sensor Means for Measuring Mechanical WindTurbine Oscillations and/or Loads are Present).

In order to return to normal production, reapplying of thrust isnecessary by pitching the rotor blades back to their operation position.

As an example of this preferred embodiment, the alternating aerodynamictorque under which the tower has been influenced caused by the suddendrop in thrust when pitching out to NOPA (state 1), the tower willoscillate with its eigenfrequency when the grid fault has recoveredresulting in excessive physical loads on the wind turbine components,especially the tower construction.

For this example a switch to state 3 has be initiated when the toweracceleration signal is within a predefined window regarding amplitudeand direction.

The pitch angle is ramped back in towards normal production pitch anglewith a maximum pitch velocity. Said pitch angle can e.g. be the value asbefore the detected grid fault event or if conditions has changed duringthe grid fault event, a new desired pitch angle. Hereby it is achievedthat a maximal dampening of said tower oscillation is obtained as wellas the rotor speed is decreased towards the rotor speed before the gridfault event.

State 13: (Final Control—if Sensor Means for Measuring Wind TurbineOscillations and/or Loads are Not Present).

In order to return to normal production reapplying of thrust isnecessary by pitching the rotor blades back to their operation positioni.e. the pitch value obtained before detecting a grid fault withpredefined control parameters e.g. fixed pitch rate.

As an example of this preferred embodiment the slope of the pitch ratecan be calculated as:

${Pitchrate} = \frac{\left( {{\theta\;{actual}} - {\theta\;{predip}}} \right)}{Trampback}$where

-   -   θactual=is the actual pitch angle    -   θpredip=is the pitch angle before the grid fault event or, if        conditions has changed during the fault event, a new desired        pitch angle.    -   Trampback=a predefined ramp back time

In one embodiment the Trampback must be defined to be longer than oneperiod of the wind turbine tower eigenfrequency in order not to causepositive interference on the tower oscillation when ramping back i.e.for a wind turbine tower with an eigenfrequency of e.g. 0.5 Hz theTrampback must be defined to be longer than 2 seconds such as up to 4seconds.

In another embodiment for another type of tower with an eigenfrequencyof e.g. 1 Hz the Trampback must be defined to be longer than 1 secondsuch as 1.5 seconds.

For an embodiment of a wind turbine tower wherein the tower is veryrigid e.g. a short tower the Trampback may be chosen to a shorter periodof time than a taller and more flexible tower such as the abovementioned embodiments.

FIG. 4 illustrates for one embodiment of the invention conditions thatare to be met for the detection of said grid fault event and therebyinitiation of a safety mode where the wind turbine is controlled by theinvented grid fault control algorithm.

Firstly (not illustrated) the recent generated power must be higher thana predefined limit. If the generated power is less, the normal controlalgorithm will be able to avoid over speeding or the rotor during a gridfault event and the wind turbine will continue operating in normal modesince there is no imminent danger for e.g. over speeding of the rotordue to the low power in the wind.

Secondly the slope of the grid voltage drop 10 must be higher than apredefined slope limit 11. The slope limit is defined by the operatingrange of the normal wind turbine controller and its ability to adapt toalternating grid voltages in order to keep control and avoid overspeeding of the rotor.

Thirdly the voltage drop must be of a certain predefined size i.e. thegrid voltage must drop to below a threshold value U_(threshold) 12.

If said three conditions are met the grid voltage is within the crossedarea 15 meaning that a utility grid fault is detected and said gridfault control algorithm is initiated.

FIG. 5 illustrates for one embodiment of the invention conditions thatare to be met for the detection of a grid recovery and thereby allowingthe control algorithm to proceed towards normal operation.

Firstly if the grid voltage rises above a predefined low voltage limitfor normal operation U_(threshold) 12 the turbine may obtain normaloperation.

Otherwise the turbine may obtain normal operation if the grid voltagerises a predefined amount U_(grid,add) above the present low voltagelevel U_(grid,minimum) during the grid fault AND the voltage is acertain predefined amount U_(converter,add) above the limitU_(converter,minimum) where the wind turbine converter is able toproduce active power.

If said conditions are met the grid voltage is within the crossed area14 meaning that the grid has recovered and said grid fault controlalgorithm is allowed to proceed towards normal production.

FIG. 6 illustrates schematically one embodiment of the invention, wheresensor means for measuring tower acceleration is present, a simplifiedtiming diagram showing the relation between control states, pitch angle,tower acceleration, tower displacement and generator rpm during a gridfault.

State 0:

The pitch angle, tower acceleration, tower displacement and generatorrpm. are ideal constant during normal operation.

State 1:

When a utility grid fault is detected 15 the wind turbine rotor blade orblades are pitched towards NOPA 16. The hereby sudden change inaerodynamical thrust is reflected by a tower acceleration anddisplacement 17, 18. Furthermore the generator rpm is increased due toan excessive amount of aerodynamical power 19.

State 2:

When said pitch angle reaches NOPA 20, yet another opposite directedchange in thrust occurs and the tower will start oscillating at itseigen frequency 21, 22. The generator rpm is here stabilized at an overspeed level 23 as there now is a balance between incoming aerodynamicpower and generated power.

State 3:

The rotor blade or blades are pitched back to operational settings 24when a grid recovery has been detected and for this embodiment state 3is entered when said tower acceleration goes negative 25 i.e. reapplyingof aerodynamical thrust is in counter phase to the tower acceleration.The result is damped tower acceleration 26 when returning to normaloperating mode (state 0), producing only a damped tower displacement 27whereby loads on the tower has been reduced. Furthermore the generatorrpm is decreased 28 ideal to the level as before entering said gridfault mode 29.

FIG. 7 illustrates schematically one embodiment of the invention, wheresensor means for measuring mechanical oscillations and/or loads are notpresent, a simplified timing diagram showing the relation betweencontrol states, pitch angle, tower acceleration and generator rpm.during a grid fault.

The description for states 0, 1 and 2 are same for this embodiment asdescribed for FIG. 6 and will not be repeated here.

State 13:

The rotor blade or blades are pitched back to operational settings 30with predefined control parameters e.g. Trampback. For this embodimentstate 13 is entered immediately after grid recovery has been detected.The pitch rate is chosen to be of a value that does not result insignificant further excessive mechanical oscillations e.g. toweracceleration 31, 32.

The result is a tower acceleration 31, 32 when returning to normaloperating mode (state 0), producing a tower displacement 33 wherebyloads on the tower are kept within allowable limits. Furthermore thegenerator rpm is decreased 34 ideal to the level as before entering saidgrid fault mode 29.

The invention described has been exemplified above with reference tospecific examples of control algorithms for a wind turbine during LVRT.However, it should be understood that the invention is not limited tothe particular examples but may be designed and altered in a multitudeof varieties within the scope of the invention as specified in theclaims e.g. with use of other algorithm states ormeasured/detected/established/estimated values.

1. A method for controlling a wind turbine connected to a utility grid,the method comprising: operating the wind turbine with operationalsettings in a first state; in response to a fault event in the utilitygrid, controlling one or more rotor blades of the wind turbine or powergenerated by the wind turbine to the utility grid to change theoperational settings of the wind turbine from the first state to asecond state that avoids overspeeding of a rotor of the wind turbine;measuring values of mechanical oscillations of the wind turbine or loadsof the wind turbine; and in response to the utility grid recovering fromthe fault event, controlling the one or more rotor blades of the windturbine or the power generated by the wind turbine to the utility gridby reapplying thrust in counter phase to the mechanical oscillations orthe loads of the wind turbine to compensate for the mechanicaloscillations of the wind turbine or the loads of the wind turbine as theoperational settings of the wind turbine are changed from the secondstate.
 2. The method of claim 1 wherein the mechanical oscillations orthe loads are detected by means located in the wind turbine.
 3. Themethod of claim 1, wherein the fault event is indicated by detection ofa grid voltage or the voltage/sec slope of an alternating utility gridvoltage.
 4. The method of claim 1 wherein the fault event comprises alow voltage grid fault.
 5. The method of claim 1 wherein the thrust isreapplied in counter phase to the mechanical oscillations, and themechanical oscillations are tower oscillations, rotor bladeoscillations, or foundation oscillations.
 6. The method of claim 2wherein the means is located in a tower, a nacelle, rotor blades, or afoundation of the wind turbine.
 7. A method for controlling a windturbine connected to a utility grid, the method comprising: operatingthe wind turbine with operational settings in a first state; in responseto a fault event in the utility grid, controlling one or more rotorblades of the wind turbine or power generated by the wind turbine to theutility grid to change the operational settings of the wind turbine fromthe first state to a second state that avoids overspeeding of a rotor ofthe wind turbine; and in response to the utility grid recovering fromthe fault event, controlling one or more rotor blades of the windturbine or the power generated by the wind turbine to the utility gridin dependency of a fixed control value by reapplying thrust with aconstant pitch rate as the operating settings of the wind turbine arechanged from the second state.
 8. The method of claim 7 wherein thefixed control value is substantially different from its extremes.
 9. Themethod of claim 7 wherein the fault event is indicated by detection of agrid voltage or the voltage/sec slope of an alternating utility gridvoltage.
 10. The method of claim 7 wherein the fixed control valuecomprises a constant pitch rate.
 11. The method of claim 10 wherein theconstant pitch rate is less than 10 degree/sec.
 12. The method of claim10 where the constant pitch rate is less than 8 degrees/sec.
 13. Amethod for controlling a wind turbine connected to a utility grid, themethod comprising: operating the wind turbine with operational settingsin a first state; in response to a fault event in the utility grid,controlling one or more rotor blades of the wind turbine or powergenerated by the wind turbine to the utility grid to change theoperational settings of the wind turbine from the first state to asecond state that avoids overspeeding of a rotor of the wind turbine;and in response to the utility grid recovering from the fault event,controlling one or more rotor blades of the wind turbine or powergenerated by the wind turbine to the utility grid in dependency of afixed control value by reapplying thrust in dependency of oscillation ofa tower of the wind turbine as the operating settings of the windturbine are changed from the second rate.
 14. The method of claim 13wherein the fixed control value is substantially different from itsextremes.
 15. The method of claim 13 wherein the fault event isindicated by detection of a grid voltage or the voltage/sec slope of analternating utility grid voltage.
 16. The method of claim 13 wherein thefixed control value comprises a constant pitch rate.
 17. The method ofclaim 16 wherein the constant pitch rate is less than 10 degree/sec. 18.The method of claim 16 where the constant pitch rate is less than 8degrees/sec. tower of the wind turbine as the operating settings of thewind turbine are changed from the second state.
 19. A method forcontrolling a wind turbine connected to a utility grid, the methodcomprising: operating the wind turbine with operational settings in afirst state; in response to a fault event in the utility grid,controlling one or more rotor blades of the wind turbine or powergenerated by the wind turbine to the utility grid to change theoperational settings of the wind turbine from the first state to asecond state that avoids overspeeding of a rotor of the wind turbine;and in response to the utility grid recovering from the fault event,controlling one or more rotor blades of the wind turbine or powergenerated by the wind turbine to the utility grid in dependency of afixed control value by reapplying thrust over a predefined time periodas the operating settings of the wind turbine are changed from thesecond state.
 20. The method of claim 19 wherein the fixed control valuecomprises a predefined time period.
 21. The method of claim 19 whereinthe fixed control value is substantially different from its extremes.22. The method of claim 19 wherein the fault event is indicated bydetection of a grid voltage or the voltage/sec slope of an alternatingutility grid voltage.
 23. The method of claim 19 wherein the fixedcontrol value comprises a constant pitch rate.
 24. The method of claim23 wherein the constant pitch rate is less than 10 degree/sec.
 25. Themethod of claim 23 where the constant pitch rate is less than 8degrees/sec.
 26. The method of claim 20 wherein the predefined timeperiod is more than 0.5 seconds.
 27. The method of claim 20 wherein thepredefined time period is more than 4 seconds.